Pressure swing adsorption (PSA) processes provide an efficient and economical process for separating a multicomponent gas stream that contains at least two gases that have different adsorption characteristics. One of the gases may be preferentially adsorbed and can be an impurity that is separated from the other gas, which may be taken off as product. Alternatively, the gas that is preferentially adsorbed can be the desired product, which is separated from the other gas. For example, it may be desired to remove carbon monoxide and light hydrocarbons from a hydrogen-containing feed stream to produce an impurity-depleted hydrogen (99+% H2) stream for a hydrocracking or hydrotreating process where the impurities, especially carbon monoxide, could adversely affect the catalyst or the reaction.
In pressure swing adsorption processes, the multicomponent gas stream is typically fed to one or more adsorption beds at an elevated pressure to effectuate adsorption of at least one component, while at least one other component passes through the adsorption bed. At a defined time, feed to the adsorption bed is terminated and the adsorption bed is depressurized in one or more cocurrent depressurization steps wherein pressure is reduced to a defined level that permits the separated, less-strongly adsorbed component or components to be withdrawn from the adsorption bed without significant desorption of the preferentially adsorbed components. Then, the adsorption bed is depressurized in a countercurrent depressurization step wherein the pressure in the adsorption bed is further reduced by withdrawing desorbed gas countercurrently to the direction in which of the multicomponent gas stream is fed. In multi-bed adsorption units, there are typically additional steps, and those noted above may be done in stages.
It is particularly desirable to minimize the amount of carbon monoxide in the impurity-depleted hydrogen streams, with residual carbon monoxide content in the impurity-depleted hydrogen streams after adsorption targeted for less than about 10 parts per million by volume. A conventional technique for controlling residual carbon monoxide content in the impurity-depleted hydrogen stream is to adjust the duration of the adsorption step for each adsorption bed. If the residual carbon monoxide content of the impurity-depleted hydrogen stream is too high, the adsorption step is shortened. Conversely, if the residual carbon monoxide content too conservative, the adsorption step may be lengthened to increase hydrogen recovery rates. However, control of residual carbon monoxide content in impurity-depleted hydrogen streams that are produced in accordance with existing control systems for pressure swing adsorption processes is difficult because residual carbon monoxide content in the impurity-depleted hydrogen streams is slow to react to changes in cycle time for the adsorption step, and because once residual carbon monoxide content in the impurity-depleted hydrogen streams begins to increase, the increase in residual carbon monoxide content is generally sharp. As a result, existing control systems that sense the residual carbon monoxide content in the impurity-depleted hydrogen stream are ineffective to adequately lengthen the adsorption step without exceeding desired residual carbon monoxide content limits in the impurity-depleted hydrogen stream. Thus, existing pressure swing adsorption processes are generally operated conservatively with residual carbon monoxide content in the impurity-depleted hydrogen stream significantly below permissible threshold values for residual carbon monoxide content. Such conservative operation of the existing pressure swing adsorption processes reduces yield of recovered hydrogen and is inefficient.
Accordingly, it is desirable to provide methods of preparing an impurity-depleted hydrogen stream, methods of analyzing content of an impurity-depleted hydrogen stream, and pressure swing adsorption apparatuses than enable maximized control of residual carbon monoxide content in the impurity-depleted hydrogen stream, thereby enabling maximized hydrogen recovery without risk of exceeding permissible threshold values for residual carbon monoxide content in the impurity-depleted hydrogen stream. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.